1. Field of the Invention
The present invention relates to a molding material for a fuel cell separator, a method for preparing the material, a fuel cell separator produced from the molding material, and a fuel cell comprising the separator.
A stack, the most important constituent unit of a fuel cell, is composed of a membrane electrode assembly and a bipolar plate called a ‘separator’. The separator is a core element acting as a passage for supplying hydrogen and oxygen to the membrane electrode assembly and transferring electrons generated by a catalytic reaction, and at the same time, playing a role in separating unit cells so as to maintain insulation between the unit cells. For these reasons, fuel cell separators are required to have basic characteristics, such as flexural strength, gas permeability and electrical conductivity, to some extent.
2. Description of the Related Art
Metal separators have been used in conventional fuel cells, but have problems of severe corrosion at contact sites with electrolytes, thus deteriorating the performance and shortening the service life of the fuel cells. Under such circumstances, separators made of carbon materials have drawn attention in terms of superior corrosion resistance. Graphite is the most representative carbon material. Graphite is superior in both corrosion resistance and chemical resistance, and shows high electrical conductivity comparable to metals. For these reasons, graphite separators are currently in the spotlight as substitutes for metal separators.
However, since graphite is difficult to process into a desired shape, it incurs much higher processing costs than the price of graphite alone, which is economically disadvantageous in terms of the overall production costs. Accordingly, there have been introduced carbon composite separators that can be molded into desired shapes while maintaining the inherent characteristics of graphite, including superior chemical resistance and electrical conductivity.
These carbon composite separators are generally produced by mixing a graphite powder and a thermosetting or thermoplastic resin, and molding the mixture via compression molding or injection molding. Graphite powders commonly used for this purpose are divided into spherical graphite powders and planar graphite powders in terms of their shape, and natural graphite powders and expanded graphite powders in terms of their preparation manner. In addition, as thermosetting resins, unsaturated polyester resins, epoxy resins, and phenol-based resins are used.
The use of spherical graphite powders or natural graphite powders in the production of fuel cell separators causes problems of low gas permeability, low electrical conductivity and poor strength characteristics. Particularly, additional use of powdery resins enables dry mixing and thus the overall procedure is advantageously simplified. However, the use of powdery resins degrades the dispersibility of the raw materials, causing heterogeneity in the finished products. Further, additives for improving the performance of the final products have extremely limited applicability. As a result, the above-mentioned problems lead to the manufacture of fuel cell stacks with low energy efficiency.